Method for obtaining synthesis gas by partial catalytic oxidation

ABSTRACT

The invention relates to a method for obtaining synthesis gas by partial catalytic oxidation, consisting in bringing a hydrocarbon in a gaseous state into contact with an oxidizing gas, and therefore possibly a small amount of water vapor, in the presence of a catalyst comprising at least one silicon carbide at a temperature of more than 800° C. According to the invention, the silicon carbide has a specific surface which is determined by the BET method and which is less than or equal to 100 m 2 /g, the contact time between the mixture of gaseous hydrocarbon, oxidizing gas and silicon carbide being more than 0.05 seconds and the pressure inside the reactor being greater than atmospheric pressure.

This invention pertains to a technique for obtaining synthesis gas withor without fire. More specifically, it involves techniques known asauto-thermal reforming or ATR and catalytic partial oxidation (CPOx)without any steam or with a limited quantity of water vapor.

The techniques for obtaining synthesis gas by reforming methane withsteam are well known in industry and have been the basis for thedevelopment of the fertilizer and methanol industries and of many majorintermediate products in the chemical industry. However, thesetechniques, which use water vapor as the principal agent of methaneoxidation, result in the creation of hydrogen-rich synthesis gas withH₂/CO molar ratios far above 2, which is a value that is soughtspecifically, when the synthesis gas is transformed into paraffinichydrocarbons with mostly —CH₂— links. In particular, an H₂/CO molarratio of 2 is necessary for the conversion of synthesis gas intohydrocarbons through the Fischer Tropsch reaction.

The replacement of the water vapor with oxygen as a methane oxidationagent theoretically results in obtaining an H₂/CO molar ratio of 2. Inpractice, however, it is difficult to use oxygen alone due to the highrisk of the formation of coke; therefore, it is still necessary to addwater vapor. This is why the ATR technique was developed. It unitespartial combustion and catalytic conversion in two successive steps inthe same reactor. However, this process uses too great a quantity ofwater vapor, which causes a drop in the equilibrium temperature of thegaseous mixture at the outlet of the reactor and thus leads to anincreased production of CO₂ to the detriment of CO.

Manufacturers are therefore still interested in a partial oxidationtechnique for methane that makes it possible to obtain an H₂/CO molarratio of 2 directly in the output gas, under feasible workingconditions, with minimal CO₂ content.

Among the techniques for producing synthesis gas through the catalyticpartial oxidation of methane, it is common to operate in the presence ofmetallic oxide-based catalysts at temperatures above 700° C., and rarelyabove 1,000° C., as described in documents U.S. Pat. Nos. 5,744,419 A,6,007,699 A, WO 85/05094 A, and EP 0 742 172 A. These catalystsgenerally contain one or more of the metals in the group comprised ofplatinum, palladium, rhodium, ruthenium, iridium, nickel, iron, andcobalt, supported by at least one refractory oxide from the groupcomprised of alumina, titanium oxide, silica, zirconia, and magnesia.However, maintaining temperatures above 700° C. throughout the processpromotes the sintering of these catalysts and the formation of carbondeposits that block the active sites of the catalyst, resulting in thedeactivation of the catalyst. While this sintering is occurring, one canalso observe a migration of the active nature of the catalyst in thesupport, which makes catalyst regeneration more difficult and morecostly, as well as the resumption of the active phase, in the case ofprecious metals in particular.

Furthermore, the implementation of these techniques of convertingmethane, or more generally, hydrocarbons into synthesis gas makes itnecessary to work with highly critical contact times between thecatalyst and the gas flow, which, because they are difficult to control,are preferably shorter than 10 ms (10 milliseconds, or 0.01 seconds).Although such requirements are not respected when the process isinitiated, since the reaction is exothermic and the heat transfer insidethe catalyst bed is insufficient, the system may potentially experiencerunaway or even explode. Understandably, manufacturers are hesitant touse these techniques, given the personal safety hazards that theseprocesses may present.

In EP 313 480 A, it is suggested that silicon carbide with a highspecific surface of greater than 100 m²/g, measured using the BETmethod, even at 1,000° C., and preferably greater than 200 m²/g,possibly supporting a metal such as tungsten or nickel, be used in thecatalyst beds for the controlled oxidation of hydrocarbons such asmethane. Such catalysts offer the advantage of resisting doublepoisoning by coke and by the metals, as the coke is not a disturbancesince the catalyst is easily regenerated, and because the metals can berecovered due to the chemical resistance of the silicon carbides used assupports. However, no advantage of any kind pertaining to theimplementation of a controlled oxidation process is described in EP 313480 A, particularly in terms of initiating the process or concerning thecharacteristics necessary for the process to function while maintainingcomplete personal safety.

In order to prevent the occurrence of explosive reactions, which aredifficult to control, the Applicant has developed a technique forproducing synthesis gas with or without fire, by which initiationoperations are facilitated through improved heat conductibility insidethe catalyst bed and a much slower gas output in the reactor in whichthe reaction occurs, thus making it possible to control the reaction,both at temperatures greater than 1,000° C. and at a temperature of 800°C., since the catalyst bed mechanically resists these temperatures.

The purpose of this invention is therefore to provide a technique forobtaining synthesis gas through catalytic partial oxidation with orwithout fire, consisting of putting a hydrocarbon in its gaseous statein contact with an oxidizing gas, as well as a limited quantity of watervapor when necessary, in the presence of a catalyst that includes atleast one silicon carbide, at a temperature above 800° C., characterizedin that the silicon carbide has a specific surface, as determined by theBET method, of less than or equal to 100 m²/g, where the contact timebetween the gaseous hydrocarbon mixture, the oxidizing gas and whenapplicable the water vapor, and the catalyst is greater than 0.05seconds, and in that the pressure inside the reactor is greater thanatmospheric pressure.

According to one important characteristic of this process, one canadjust, first, the reaction temperature to a value of between 800° C.and 1,400° C., and second, the contact time of the gaseous hydrocarbonmixture, the oxidizing gas, and when necessary the water vapor, with thecatalyst between 0.05 and 5 seconds.

In addition to the advantages related to the actual nature of one partof the catalyst, the technique described in this invention offers theadvantage of operating under non-critical conditions, particularlyconcerning the contact time of the gaseous mixture (hydrocarbon gas,oxidizing gas, and water vapor) with the catalyst. Also, the catalystsupport is a perfect conductor for the heat produced when the reactionis initiated, which makes it possible to prevent the runaway phenomenaobserved by applying the prior art. This good heat conductivity in thesupport makes it possible to maintain proper homogeneity of the catalysttemperature in the bed, and therefore to avoid the formation of hotpoints that generate coke, which promotes a sintering phenomenon in theactive phase, as observed in earlier techniques. This makes it possibleto use a single bed in a traditional reactor and thus to avoid the useof a tube reactor, which is always difficult to fill uniformly withgrains of catalyst powder. Another advantage is the ability to increasethe temperature of the catalyst bed, which makes for improved conversioninto synthesis gas, whether the reaction takes place in the presence orin the absence of water vapor, in the context of an application in ATRunits or catalytic POx.

In the technique described in this invention, the oxidizing gas maycontain more than 20% oxygen by volumes and preferably between 40% and100% oxygen by volume.

In order to implement the technique described in this invention, a molarratio of the carbon in the hydrocarbon (C) to the oxygen that approachesthe stoichiometry of the reaction for obtaining synthesis gas, or morespecifically, a value varying within a range between 1.6 and 2.6, willbe chosen.

Contrary to the specifications from prior art, the catalyst containsmore than 50% silicon carbide by weight, with a BET-determined specificsurface, which according to the French Standard (NF) X11-621, is lessthan 100 m²/g, in particular between 15 and 80 m²/g, and preferablybetween 20 and 40 m²/g. These silicon carbides also have a mesoporositydetermined by the nitrogen BET method, according to the standard NFX11-621, of between 20 and 100 mn, and a macroporosity determined by amercury porosity measurement of between 5 and 100 μm.

Preferably, the silicon carbide used as a support is solid and comprisedof formed or unformed grains or of solid foam. By formed grains, what ismeant is grains in the form of carbide balls, extruded grains in theform of cones, trilobes or otherwise, or monoliths in the form of disksor conic sections.

In a preferred method of implementing the technique described in thisinvention, the pressure in the reactor can be maintained at a value ofbetween 2×10⁵ and 150×10⁵ Pa, and preferably between 5×10⁵ and 80×10⁵Pa.

The silicon carbide in the catalyst can serve as a support for themetals in Group VIII. The metal content of Group VIII is between 0.5%and 20% of the weight of the catalyst, and preferably between 1% and 10%of the catalyst weight. The preferred Group VIII metal is nickel.

The catalyst can be obtained by any of the methods known to an expert inthe field. In particular, it can be prepared by impregnating the siliconcarbide using a solution containing a salt of at least one metal fromGroup VIII, where the impregnated metallic salt is subsequentlydecomposed by calcination in air at between 300° C. and 400° C. of theimpregnated support. In order to implement the technique described inthe invention, the catalyst can be used as is, or reduced in situ in thereactor under a hydrogen current, at a temperature of between 200° C.and 400° C., before turning the unit on.

When the technique described in the invention is initiated, in order toavoid uncontrollable runaway problems with the controlled oxidationreaction, the contact time between the gaseous hydrocarbon mixture, theoxidizing gas and when applicable, the water vapor, will be adjusted toa value of between 0.5 and 5 seconds.

Favorably, in this invention, the quantity of steam added will belimited to a water vapor-to-hydrocarbon carbon (H₂O/C) molar ratio ofless than or equal to 0.2.

In a preferred method of implementing this invention, the temperature ofthe catalyst bed is maintained between 900° C. and 1,300° C. The higherthe temperature, the more CO formation is promoted with respect to theformation of CO₂, which results in an increased overall effectiveness ofthe subsequent steps of the synthesis gas conversion process, such asthe Fischer Tropsch conversion or methanol production.

However, when it is necessary to work under high pressures of above25×10⁵ Pa, it is preferable to operate in at least two successive steps:an initial step to condition the silicon carbide under pressure ofbetween 5×10⁵ and 10×10⁵ Pa for at least two hours, and then a secondstep under pressure of between 25×10⁵ and 80×10⁵ Pa.

In one preferred method of implementing the invention, in order to makethe reaction self-igniting, the gases are preheated to between 400° C.and 650° C. before they are put in the catalyst bed.

The hydrocarbons enabling the formation of synthesis gas are chosen fromamong gasoline, deposit gas condensates, and hydrocarbons with 1 to 3carbon atoms, preferably methane.

As described above, the technique described in the invention can beimplemented in existing ATR units.

Examples will be described below to illustrate the invention, withoutlimiting its scope.

Two figures are appended to illustrate Example 3:

FIG. 1 represents two partial oxidation curves for methane on the γalumina-based catalyst (C₁) under the conditions of Example 3 for twocontact times, 0.6 and 3 seconds respectively; and

FIG. 2 represents a partial oxidation curve for methane on the SiC-basedcatalyst (C₂) under the conditions of Example 3 for two contact times,0.6 and 3 seconds respectively.

FIG. 3 represents a partial oxidation curve for methane on the aalumina-based catalyst under the conditions of Example 3 for a contacttime of 3 seconds.

EXAMPLE 1

This example will attempt to illustrate the effectiveness of the siliconcarbide-based catalytic supports with a BET specific surface of lessthan 100 m²/g and, more specifically, of between 15 and 80 m²/g.

A 10-g sample of silicon carbide (SiC) in the form of 0.4- to 1 -mmgrains, with a BET specific surface of 40 m²/g is used in this example.It was previously impregnated with a nickel nitrate solution, enablingthe deposit of 2.6 g of salt on the SiC to obtain a final nickel contentof 5% by weight with respect to the final catalyst. The impregnatedcatalyst is dried in 100° C. air, and then calcinated at 300° C. in dryair.

In a microreactor, 2 g of SiC-supported catalyst is added, and then theatmosphere in the reactor is purged with airflow at room temperature.The pressure in the reactor is then raised to 5×10⁵ Pa in a mixture ofmethane and air with a carbon/oxygen molar ratio of 2.6, while thetemperature in the catalyst bed is 900° C. The circulation speed of thegaseous mixture is adjusted for a contact time of 3 seconds, and themixture at the outlet of the reactor is then analyzed online bychromatography in the gaseous phase.

The conversion results and the H₂/CO molar ratios are provided in Table1 below. TABLE 1 HC/CO Conversion Reaction Time (h) (% by weight) H₂/COMolar Ratio 1 75.0 2.2 2 74.2 2.1 3 74.6 2.1 4 75.8 2.0 24 75.1 2.1 4875.4 2.1 72 73.8 2.0 96 75.7 2.1

These results comply with what was expected, which was an H₂/CO molarratio of approximately 2 and a conversion rate of more than 70%.

When a shorter contact time was used, less than 0.5 seconds, theconversions and the molar ratios were comparable.

The properties of these results confirm the advantage of the technique,which makes it possible to more easily implement (longer contact time)the catalyst partial oxidation of methane or any other condensate, witha single catalyst bed.

Likewise, we have not observed any deactivation of the catalyst.

EXAMPLE 2

In this example, we operate with the same catalyst as in Example 1, butwith a gas mixture with a methane/air ratio of 2 (or CH₄/O₂=2). Theresults are provided in Table 2 below. TABLE 2 HC/CO Conversion ReactionTime (h) (% by weight) H₂/CO Molar Ratio 1 90.1 2.1 2 89.5 2.0 3 90.32.0 4 90.5 2.1 24 89.3 2.0 48 90.0 2.1 72 89.6 2.0 96 89.0 2.0

By operating with an HC/O₂ ratio of 2, by comparison to Example 1, anincrease in the conversion of nearly 90% is obtained, so that the H₂/COratio at the outlet remains unmodified at a value of 2.

EXAMPLE 3

The purpose of this example is to show the ability of the siliconcarbide-based support to rapidly diffuse the heat produced during themethane oxidation reaction outside the reaction zone, thus preventingrunaway in the system temperature.

In this example, two catalysts, each with the same 5% nickel content byweight; are tested: one, C₂, is on a SiC support (grains 6 mm long and 2mm in diameter), with a specific surface equal to 25 m²/g, while theother (C₁) is gamma alumina-based with a high specific surface of 224m²/g. The supports are impregnated according to the same operationalmethod as the one described in Example 1. The controlled methaneoxidation reaction is conducted under the following conditions: reactiontemperature: 900° C.; CH₄/O₂ ratio: 2.6; total pressure: 5×10⁵ Pa;contact time: 3 seconds. The results obtained are presented in FIG. 1for C₁ and in FIG. 2 for C₂, for an identical test time.

With the γ alumina-based catalyst C₁, the heat produced during thereaction is extremely significant, causing a total conversion thatlargely exceeds the one produced by thermodynamics (FIG. 1). By takinginto account the thermodynamic data, the excess temperature produced inthe first hours of the test on the alumina exceeds a few hundreddegrees. In this situation, it is probable that the methane decomposesessentially into carbon and hydrogen. Then, as the quantity of carbondeposited on the catalyst increases, the catalyst increasingly becomes aconductor and the complete conversion returns to the level expected fora reaction temperature of 900° C.

With the SiC-based catalyst C₂, a conversion runaway upon initiation ofthe reaction did not occur (FIG. 2), due to the high thermalconductivity of the support, which quickly evacuated the heat formedoutside of the reaction zone.

The comparison of the thermal behavior of these two catalysts once againconfirms the superiority of a SiC-based catalyst compared to analumina-based catalyst, since the first diffuses the heat produced bythe reaction more rapidly during the initiation phase.

We also noted that the quantity of carbon deposited on the catalyst islower for catalyst C₂ than for catalyst C₁. Furthermore, there is adeterioration of the structure of the γ alumina-based catalyst: thespecific surface drops to 62 m²/g, and the mechanical resistancedecreases (a part of the support in extruded form was fragmented intosmaller grains).

In order to eliminate the carbon and evaluate the possibilities ofregenerating these catalysts, the spent catalysts C₁ and C₂ weresubjected to an oxidizing regeneration in air. This regenerationconsists of increasing the temperature of the reactor from roomtemperature to a temperature of 900° C., with an increase rate of 10°C./min, then maintaining this temperature for 30 minutes, in order toburn the carbon. After cooling, it was noted that alumina-based catalystC₁ did not resist the oxidizing regeneration treatment and wascompletely destroyed, while SiC-based catalyst C₂ retained its initialtexture.

By using an α alumina-based catalyst with a specific surface of 5 m²/g,under the same operating conditions with a contact time of 3 seconds,the results obtained are comparable to those obtained with a γ aluminabase (see FIG. 3) with grain rupture.

1. Technique for obtaining synthesis gas by catalytic partial oxidation,consisting of putting a hydrocarbon in its gaseous state in contact withan oxidizing gas, and also a small amount of water vapor when necessary,in the presence of a catalyst that includes a silicon carbide, at atemperature greater than 800° C., characterized in that the siliconcarbide has a specific surface, determined by the BET method, of lessthan or equal to 100 m²/g, where the contact time between the gaseoushydrocarbon mixture, the oxidizing gas and when necessary the watervapor, and the catalyst is longer than 0.05 seconds, and in that thepressure inside the reactor is greater than atmospheric pressure. 2.Technique described in claim 1, characterized in that the reactiontemperature is between 800 and 1,400° C.
 3. Technique described inclaims 1 and 2, characterized in that the contact time of the gaseoushydrocarbon mixture, the oxidizing gas, and when necessary the watervapor with the catalyst is between 0.05 and 5 seconds.
 4. Techniquedescribed in claims 1 to 3, characterized in that the oxidizing gascontains more than 20% oxygen by volume, and preferably between 40% and100% oxygen by volume..
 5. Technique described in claims 1 to 4,characterized in that the molar ratio of the carbon from the hydrocarbonto the oxygen varies between 1.6 and 2.6.
 6. Technique described inclaims 1 to 5, characterized in that the catalyst contains more than 50%silicon carbide by weight, with a BET specific surface of between 15 and80 m²/g and, preferably of between 20 and 40, m²/g.
 7. Techniquedescribed in claims 1 to 6, characterized in that the silicon carbide insaid catalyst has a mesoporosity as determined by the nitrogen BETmethod of between 20 and 100 nm, and a macroporosity determined by themercury porosity measurement of between 5 and 100 μm.
 8. Techniquedescribed in claims 1 to 7, characterized in that the silicon carbide issolid and comprised of formed or unformed grains or of solid foam. 9.Technique described in claims 1 to 8, characterized in that the pressurein the reactor is between 2×10⁵ and 150×10⁵ Pa, and preferably between5×10⁵ and 80×10⁵ Pa.
 10. Technique described in claims 1 to 9,characterized in that the catalyst includes 0.5% to 20% by weight of ametal from Group VIII, and preferably 1% to 10% metal by weight. 11.Technique described in claim 10, characterized in that the metal fromGroup VIII is nickel.
 12. Technique described in claims 1 to 11,characterized in that the contact time between the gaseous hydrocarbonmixture, the oxidizing gas, and when necessary water vapor is between0.5and 5 seconds.
 13. Technique described in claims 1 to 12,characterized in that the water vapor is added in a watervapor-to-hydrocarbon carbon (H₂O/C) molar ratio of less than or equal to0.2.
 14. Technique described in claims 1 to 13, characterized in thatthe temperature of the catalyst bed is maintain between 900° C. and1,300° C.
 15. Technique described in claims 1 to 14, characterized inthat it includes an initial step for conditioning the silicon carbideunder pressure of between 5×10⁵ and 10×10⁵ Pa for at least two hours,and then a second step under pressure of between 25×10⁵ and 80×10⁵ Pa.16. Technique described in claims 1 to 15, characterized in that thegases are preheated to between 400° C. and 500° C. before being put incontact with the catalyst.
 17. Technique described in claims 1 to 16,characterized in that the hydrocarbons are chosen from among gasoline,deposit gas condensates, and hydrocarbons containing 1 to 3 atoms ofcarbon, and preferably methane.